Add Club.website; rebuild Colours per D9's exact markup; add a mobile app preview

- club.website (URLField, blank) editable from both the club's own Identity
  page and the platform control panel's club form.
- Colours card rebuilt from the design canvas's literal D9 markup rather
  than the prior in-between version: swatch + mono hex merged into one
  bordered row (the swatch input and hex text input are the same real form
  field, just laid out together), and a contrast-check row of two colour-
  filled boxes instead of a plain text list -- labelled dynamically
  ("Black"/"White" on primary/secondary) since a club can pick a colour
  light enough that black, not white, is the real computed text colour.
- Logo card restyled to D9's dashed drop-zone look.
- Live preview gains a mobile app mock (marked "Coming soon") alongside the
  existing sidebar mock, using the same primary/secondary colours, ahead of
  that surface actually being built.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ECGMEwrc2k4D8VQuwjstj9

controlpanel/forms.py

@@ -14,7 +14,7 @@ from .services.admins import find_member_by_email
 class ClubForm(forms.ModelForm):
     class Meta:
         model = Club
-        fields = ["name", "legal_name", "contact_email", "slug", "sport_type", "logo", "primary_color", "secondary_color", "season_start", "season_duration_months"]
+        fields = ["name", "legal_name", "contact_email", "website", "slug", "sport_type", "logo", "primary_color", "secondary_color", "season_start", "season_duration_months"]
         help_texts = {"slug": _("Drives the club's subdomain. Left blank, it is derived from the name.")}
         # Deliberately a text input, not <input type="color">: a colour picker cannot
         # express "no colour" -- it would submit #000000 for every club that never